Image sensor having integrated thin film infrared filter

ABSTRACT

An image sensor is disclosed. The image sensor includes a plurality of pixels formed in a semiconductor substrate, each pixel including a light sensitive element. Further, a multilayer stack is formed over the pixels, the multilayer stack adapted to filter incident light in the infrared region. Finally, micro-lenses are formed over the multilayer stack and over the light sensitive element.

TECHNICAL FIELD

[0001] The present invention relates to image sensors, and moreparticularly, towards an image sensor that incorporates a thin filminfrared filter.

BACKGROUND

[0002] Image sensors are electronic integrated circuits that can be usedto produce still or video images. Solid state image sensors can beeither of the charge coupled device (CCD) type or the complimentarymetal oxide semiconductor (CMOS) type. In either type of image sensor, alight gathering pixel is formed in a substrate and arranged in atwo-dimensional array. Modern image sensors typically contain millionsof pixels to provide a high resolution image. An important part of theimage sensor are the color filters and micro-lens structures formed atopof the pixels. The color filters, as the name implies, are operative, inconjunction with signal processing, to provide a color image. Themicro-lenses serve to focus the incident light onto the pixels, and thusto improve the fill factor of each pixel.

[0003] An infrared (IR) filter is typically used in conjunction withimage sensors. The IR filter prevents or limits IR radiation from beingincident to the image sensor. Many prior silicon-based pixels arereactive to light in the IR region. Thus, if IR radiation is incident onthe pixel, the pixel will generate an output signal. This is undesirablein most applications of image sensors, since these image sensors aredesigned to provide an image based upon light visible to the human eye.

[0004] One common prior art method for solving this problem is toprovide a discrete IR filter component in front of the image sensorand/or the lens of the image sensor. The IR filter may come in variousforms, but may take the form of a coated glass filter. However, thisadds to the cost of the entire camera apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a prior art cross sectional view of a portion of animage sensor.

[0006]FIG. 2 is a top view of an image sensor showing pixels arranged ina two dimensional array and with micro-lenses formed thereon.

[0007] FIGS. 3 is a cross sectional of a semiconductor substrateillustrating an image sensor formed in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION

[0008] The present invention relates to an image sensor that has anintegrated IR filter. In the following description, numerous specificdetails are provided to provide a thorough understanding of theembodiments of the invention. One skilled in the relevant art willrecognize, however, that the invention can be practiced without one ormore of the specific details, or with other methods, components, etc. Inother instances, well-known structures or operations are not shown ordescribed in detail to avoid obscuring aspects of various embodiments ofthe invention.

[0009] Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0010]FIG. 1 shows a prior art cross-sectional simplified diagram of animage sensor 101 having micro-lenses formed thereon. As seen in FIG. 1,the image sensor includes a plurality of pixels that have lightdetecting elements 103 formed in the substrate. The light detectingelements 103 may be one of several types, such as a photodiode, aphotogate, or other solid state light sensitive element. Formed atop ofeach pixel is a micro-lens 105. The micro-lens 105 focuses incidentlight onto the light detecting elements 103. Micro-lenses are oftenformed by spin coating a layer of micro-lens material onto a planarizedlayer. The micro-lens material is then etched to form cylindrical orother shaped regions that are centered above each pixel. Then, themicro-lens material is heated and reflowed to form a convexhemispherical micro-lens. Moreover, in the region between the lightdetecting elements 103 and the micro-lens 105, denoted by referencenumeral 107, there are various intervening layers that would typicallyinclude the color filter layers 109 and various metal conducting lines.It can be appreciated that the structure of FIG. 1 is merely one exampleof an image sensor structure and that the present invention is adaptableto any number of variants. For example, the micro-lenses may be concavein nature as disclosed in my co-pending applications. Alternatively, thecolor filters 109 may be formed atop of the micro-lenses 105.

[0011]FIG. 2 shows a top view of an image sensor 201. The image sensor201 includes a plurality of pixels 203 typically arranged in a twodimensional array. In the example shown in FIG. 2, the image sensorshows a three by three array of pixels 203, though it can be appreciatedthat an actual mage sensor 201 would have many more pixels, arranged inperhaps over a thousand rows and/or a thousand columns. Further,although FIG. 2 shows the pixels in ordered columns and rows, the pixelsmay be arranged in any type of ordered arrangement. For example,alternating rows may have their pixels slightly offset from each otherlaterally in a checkerboard format.

[0012] The pixels 203 typically include a light sensitive element, suchas a photodiode or a photogate as two examples. However, it can beappreciated that other types of light sensitive elements, now known ordeveloped in the future, may be used. Further, the pixels 203 will alsoinclude amplification and/or readout circuitry. For clarity, thiscircuitry is not shown in FIG. 2. In one embodiment, the pixels 203 maybe active pixels, commonly known in the prior art. Formed atop of eachpixel 203 is a micro-lens 205.

[0013] Additionally, associated with each pixel 203 is a color filter207. The color filter 207 may be placed either between the micro-lens205 and the light sensitive element, or alternatively, be formed atop ofthe micro-lens 205. The color filter 207 is typically a pigmented ordyed material that will only allow a narrow band of light to passtherethrough, for example, red, blue, or green. In other embodiments,the color filter may be cyan, yellow, or magenta. These are but examplecolors for the color filters 207 and the present invention is meant toencompass a color filter 207 having any color. While the use ofpigmented or dyed color materials is the most prevalent form of colorfilters, other reflective type color filters may be used, such as amultilayer stack reflective material. The formation of color filters 207is known in art and will not be described herein to avoid anyunnecessary obscuration with the description of the present invention.

[0014] For example, U.S. Pat. Nos. 6,297,071, 6,362,513, and 6,271,900show the current state of the color filter art. Not shown in the topview of FIG. 1 is the IR filter incorporated into the image sensor 201.

[0015]FIG. 3 is a cross sectional view taken along line A-A of FIG. 2which shows the IR filter 301 of the present invention. As seen, asemiconductor substrate 301 has a plurality of light sensitive elements303 (associated with the pixels 203 of FIG. 2) formed therein. FIG. 3shows the light sensitive element 303 as a photodiode, though othersubstitutes and equivalents may be used. Details of forming thephotodiode and other associated circuitry are known in the prior art andwill not be repeated herein to avoid obscuring the present invention.However, examples of the prior art may be seen in U.S. Pat. No.5,904,493 and U.S. Pat. No. 6,320,617.

[0016] According to one embodiment, after the pixels 203 are formed inthe substrate, an optically transparent (in at least a portion of thevisible spectrum) base material 305 is formed over the substrate 301.The base material 305 may be formed using a blanket deposition processand may be various forms of silicon dioxide, such as thermal oxide,chemical vapor deposition (CVD) oxide, or spin on glass. The basematerial 305 may also be thought of as an interlayer dielectric.

[0017] Next, a multilayer stack 307 is deposited over the base material305 and serves as an IR filter. In one embodiment, the multilayer stack307 is adapted to reflect wavelengths in the 660-800 nm region andabove. The multilayer stack 307 consists of a plurality of pairs oftitanium oxide/silicon dioxide thin films. In one embodiment, 6 to 8pairs of titanium oxide/silicon dioxide thin films comprise themultilayer stack 307, though more or fewer pairs would be possible,dependent upon the extent to which IR radiation needs to be reflected.With a higher number of pairs, a higher reflectance is achieved, thoughat added cost.

[0018] Further, the thickness of the thin film layers is to a largeextent dependent upon the center wavelength to be reflected (i.e.rejected). Generally, the thickness of each thin film should be on theorder of one-quarter of the center wavelength. Thus, to reject thecenter wavelength of 700 nm, the thin films of titanium oxide andsilicon dioxide should be about 175 nm thick or 0.175 microns. Further,it should be noted that other materials may be used to form the pairs ofthin films and the present invention should not be limited to titaniumoxide/silicon dioxide thin films. In an alternative embodiment, thepairs of thin films may include an interlayer between the first andsecond films to increase thermal stability and to preventinterdiffusion. For example, the interlayer may be carbon.

[0019] Next, still referring to FIG. 3, a capping layer 309 is depositedonto the multilayer stack 307. The capping layer 309 is on the order ofone micron thick. It should be noted that the capping layer 309 isoptional. The capping layer 307 is useful in protecting the surface ofthe multilayer stack 307 during subsequent processing steps.

[0020] Next, color filters 311 are formed above the capping layer 307.The color filters 311 are formed using conventional processes. In theembodiment shown in FIG. 3, the color filters 311 are of the red, green,and blue coloration, but can be also cyan, yellow, and magenta. Finally,micro-lenses 313 are formed above the color filters 311. Similarly, themicro-lenses 313 can be formed using conventional processes, and may beconvex as shown in FIG. 3 or other shape.

[0021] Several features of the present invention should be noted. First,an IR filter is implemented as a multilayer stack 307.Characteristically of multilayer stacks 307, they are reflective innature and therefore deflect incident IR radiation away from the pixel.This is in contrast to absorptive pigment or dye based filters, whichstill may produce noise in the IR band and may not be as reliable.Second, in one embodiment, the multilayer stack 307 is formed prior toformation of the color filters. In general, when titanium oxide/silicondioxide thin film pairs are deposited, this requires a relatively highheat, which may damage color filters 311 (which are typically made fromorganic materials). However, should later processes for making colorfilters 311 prove to be heat resistant, the multilayer stack 307 may beformed after (e.g. atop of) the color filters 311. The same holds truefor the micro-lenses 313. Third, by integrating an IR filter onto theimage sensor, the component count, and thus cost, of the image sensorcan be reduced.

[0022] From the foregoing, it will be appreciated that specificembodiments of the invention have been described herein for purposes ofillustration, but that various modifications may be made withoutdeviating from the spirit and scope of the invention. Accordingly, theinvention is not limited except as by the appended claims.

I claim:
 1. An image sensor comprising: a plurality of pixels formed ina semiconductor substrate, each pixel including a light sensitiveelement; a multilayer stack formed over said plurality of pixels, saidmultilayer stack adapted to filter incident light in the infraredregion; a plurality of micro-lenses formed over said multilayer stackand over said light sensitive element.
 2. The image sensor of claim 1further including a color filter formed over each pixel, said colorfilter formed between said micro-lens and said multilayer stack.
 3. Theimage sensor of claim 1 further including a color filter formed overeach pixel, said color filter formed over said micro-lens.
 4. The imagesensor of claim 1 wherein the multilayer stack comprise pairs oftitanium oxide/silicon dioxide thin films.
 5. The image sensor of claim4 wherein said multilayer stack comprise 6 to 8 pairs of titaniumoxide/silicon dioxide thin films.
 6. The image sensor of claim 4 whereineach thin film is approximately 200 nm thick.
 7. A pixel of an imagesensor comprising: a light sensitive element formed in a semiconductorsubstrate; a multilayer stack formed over light sensitive element, saidmultilayer stack adapted to filter incident light in the infraredregion; a micro-lens formed over said multilayer stack and over saidlight sensitive element.
 8. The pixel of claim 7 further including acolor filter formed between said micro-lens and said multilayer stack.9. The pixel of claim 7 further including a color filter formed oversaid micro-lens.
 10. The pixel of claim 7 wherein the multilayer stackcomprise pairs of titanium oxide/silicon dioxide thin films.
 11. Thepixel of claim 10 wherein said multilayer stack comprise 6 to 8 pairs oftitanium oxide/silicon dioxide thin films.
 12. The pixel of claim 10wherein each thin film is approximately 175 nm thick.